We reveal a new regime of nonlinear frequency upconversion, where bright high harmonic X-ray supercontinua with photon energies >1.6 keV (<7.7 Å) can be produced from a tabletop femtosecond laser [1-3]. This represents the most extreme >5001 order nonlinear optical process known, and the most extreme phase-matched nonlinear process for electromagnetic waves in the 50 year history of nonlinear optics. In a remarkably favorable convergence of nonlinear optical phenomena, the multi-atmosphere gas pressures that enable bright X-ray generation also self-confines the laser beam, further enhancing the X-ray yield. The generated X-ray beams exhibit high spatial coherence, despite the very high density of the gas where the recolliding electrons responsible for high harmonic generation encounter other atoms during their free excursion. Finally, we present a generalized picture of phase matching of high harmonic generation that combines microscopic quantum physics with macroscopic extreme nonlinear optics. Counterintuitively, to generate the brightest harmonics, the order of the nonlinearity must dramatically increase from ~11 in the vacuum UV, to >5001 in the keV X-ray region. The emission evolves from a single harmonic in the UV, to a broad X-ray supercontinuum spanning thousands of harmonics in the soft X-ray region that in theory could support single cycle X-ray pulses as short as 2.5 as.
X-rays are powerful probes of the nanoworld. They penetrate thick samples and, by virtue of their short wavelength, image small objects. Moreover, using elemental absorption edges, they can provide element and chemical species-specific information. Ultrafast x-rays can capture all dynamics relevant to function at the nanoscale – even at the level of electrons. This talk will also highlight how ultrafast x-rays to capture the coupled motions of charges, spins, phonons and photons that underlie how materials and molecular systems work on the fastest timescales.[4,5]
1. T. Popmintchev, et al, “Bright Coherent Attosecond Kiloelectronvolt X-ray Supercontinua”, submitted (2012).
2. T. Popmintchev, et al, The Attosecond Nonlinear Optics of Bright Coherent X-Ray Generation, Nature Photonics 4, 822 (2010).
3. M.C. Chen et al., Bright, Coherent, Ultrafast Soft X-Ray Harmonics Spanning the Water Window from a Tabletop Light Source, PRL 105, 173901 (2010).
4. X. Zhou X, P. Ranitovic, C. Hogle, J. Eland, H. Kapteyn, M. Murnane MM, "Probing and Controlling Non-Born-Oppenheimer Dynamics in Super-Excited Triatomic Molecules”, to be published in Nature Physics (2012).
5. S. Mathias, et al, “Probing the timescale of the exchange interaction in a ferromagnetic alloy”, submitted (2012).
